1. Field of the Invention
This invention relates to a malfunction detecting system of an engine cooling apparatus, more particularly to a malfunction detecting system of a radiator, still more particularly to a malfunction detecting system of a radiator thermostat.
2. Description of the Related Art
The internal combustion engine of a vehicle is equipped with a radiator (cooling apparatus) for cooling a coolant. The radiator is connected midway of a coolant communicating passage composed of an inlet pipe and an outlet pipe. A thermostat (shut-off valve) is installed in the communicating passage. The thermostat closes the communicating passage when the coolant temperature is low, such as just after engine starting, and opens it as the coolant temperature rises so that the coolant can enter the radiator to be cooled.
Since the radiator is one of the on-board components of a vehicle, it is preferably checked for malfunction. It was for this purpose that the assignee developed a system that first checks whether the engine is in a state cooled to a temperature equal to the outside air temperature (intake air temperature) owing to thorough soaking (long-period or sufficient standing) and whether change in the outside air temperature since engine starting is small, and then, when these conditions are met, determines that the conditions for execution of malfunction detection have been established, whereafter it carries out a calculation for estimating the coolant temperature and determines that the radiator, more precisely the radiator thermostat, has malfunctioned if, for example, the detected coolant temperature has not reached the judge-normal value when the estimated coolant temperature reaches the judge-malfunction value. This technology is described in the assignee's Japanese Laid-Open Patent Application 2000-008853.
The conventional system that discriminates malfunction using the engine coolant temperature and outside air temperature utilizes the intake air temperature indicated by the output of a temperature sensor installed downstream of the throttle valve as the outside air temperature. In other words, the conventional system uses the raw output value of the intake air temperature sensor as the outside air temperature, notwithstanding that these are essentially different and that the state of engine cooling is more affected by the outside air temperature than the intake air temperature.
As shown in FIG. 9, the engine coolant temperature, intake air temperature and outside air temperature all become equal after a sufficient amount of time has elapsed following engine shutdown. Immediately after shutdown, however, the difference between the coolant temperature and the intake air temperature is large, and the difference between the intake air temperature and the outside air temperature is also large. As the graph shows, the temperatures converge with passage of time and finally become the same. When the outside air temperature is detected from the output of a sensor located in the engine compartment, however, the state of engine cooling cannot be accurately ascertained until the temperatures become equal.